The present invention relates to a photomultiplier tube, and particularly to a photomultiplier tube capable of providing improved precision in position detection when it is used as a position detector.
Photomultiplier tubes generally are so constructed that photoelectrons generated from the photocathode as it is struck by light are progressively multiplied by a plurality of dynodes to produce intensification of incident light, which is picked up by the anode or output electrode.
A convertional photomultiplier tube employing mesh dynodes is shown in FIGS. 1(A) and 1(B). FIGS. 1(A) and 1(B) are respectively top and cross-sectional views of the conventional photomultiplier tube. The numerals 1, 2, 3 and 4 represent incident light, the photocathode, mesh dynodes and the anode, respectively. In the photomultiplier tube shown in FIGS. 1(A) and 1(B), the mesh dynodes 3 are provided in successive plural layers, in the direction of an electron stream. For example the photocathode is connected to the ground potential and a voltage of about 300 volts is applied to the first dynode. The anode is divided into segments each having a larger size than the openings of each mesh dynode so that output can be picked up from the anode segment which corresponds to the area of the photocathode where it is struck by incident light.
When light 1 is incident upon the photocathode 2 in the photomultiplier tube shown in FIG. 1(B), photoelectrons are emitted from the photocathode 2 and bombarded against each dynode 3 in turn, so that secondary electrons are emitted with multiplication at each bombardment to produce an intensified output which is picked up from the anode 4. The problem with the conventional photomultiplier tube shown in FIGS. 1(A) and 1(B) is that since the electric field applied between the photocathode and a dynode is similar to that generated between plane-parallel electrodes, an electron stream emitted from one point on the photocathode spreads progressively as it is bombarded against each dynode on account of both the energy of photoelectrons and the cosine distributed emission angle thereof. This spread in the electron stream will make it impossible to precisely determine the incident position of light on the photocathode based on an output signal from the anode segment which is in correspondence to the area of the photocathode.
As shown in FIGS. 2 and 3, the same phenomenon occurs between dynodes and between the last dynode and the anode as well; the electron stream emitted from one point on the photocathode and which is in the process of multiplication will diverge progressively to produce a pyramid-like spread on account of both the energy of secondary electrons and the cosine distributed emission angle thereof. This beam divergence also makes it impossible to precisely determine the incident position of light on the photocathode based on an output signal from the anode segment which corresponds to the area of the photocathode.
FIG. 4 is a graph showing the crosstalk that occurs as a result of the electron beam divergence as described above. The horizontal axis of the graph plots the position on the photocathode where it is scanned with a very small spot of light and the vertical axis plots the relative value of the output picked up from a certain anode segment. The crosstalk occurring in the output is indicated by the hatched area. As is clear from FIG. 4, electron beam divergence causes a crosstalk to occur over a broad range of the output.
Several ideas have been proposed to prevent the occurrence of crosstalk due to electron beam divergence and three typical approaches are described below:
(1) a plurality of tubular transmission-type dynodes are used to mechanically restrict the path or orbit of electrons as described in U.S. Pat. No. 3,062,962; PA1 (2) a position sensitve photomultiplier tube in which the distance between the photocathode and the first dynode or the distance between adjacent dynodes is sufficiently decreased to increase the field strength so that the divergence of an electron stream is supressed to perform position detection; and PA1 (3) a plurality of small photomultiplier tubes are placed side by side.
The first approach employing a plurality of tubular transmission-type dynodes has the problem that gases might occur as a result of electron bombardment against the tubular electrodes. Furthermore, the need to arrange tubular electrodes in layers not only introduces considerable difficulty in the device fabrication process but also increases the dead space of the final product.
The second approach in which the distance between the photocathode and the first dynode or the distance between adjacent dynodes is sufficiently decreased to increase the field strength so that the divergence of electron stream is reasonably suppressed for position detecting purposes, is also defective in that precise position detection is hardly achieved because a substantial amount of crosstalk occurs as illustrated in FIG. 4. In addition, the distance between dynodes cannot be decreased indefinitely for the purpose of restricting the divergence of electron beam because the electric field to be applied between the dynodes without breaking down is limited, and therefore has a limiting value.
The third approach employing a plurality of small photomultiplier tubes arranged side by side increases the dead zone between tubes and the overall cost of the device is increased because of the need to employ a large number of tubes.